Monoliyhic polymeric product

ABSTRACT

Polymeric monoliths having high stiffness and strength can be produced by heating an assembly of polymer fibres under a contact pressure to a temperature at which a proportion of the fibre is selectively melted and then compressing the assembly. Preferably at least 5% of the polymer is melted so that on compression the molten materials fills the voids within the assembly. The use of polyolefin fibres especially melt spun polyethylene fibres is preferred. The products are useful e.g. as orthodontic brackets, bone prostheses and in body armour.

This is a Rule 60 Division of application Ser. No. 08/315,680 now U.S.Pat. No. 5,628,946 filed Sep. 30, 194, which is a Rule 62 Continuationapplication of Ser. No. 07/934,500, filed Oct. 21, 1992, now abandoned.

This invention relates to processes for the production of polymer sheetmaterials from oriented polymer fibres and to the products of suchprocesses.

BACKGROUND OF THE INVENTION

One method which is widely used to produce high modulus polymer sheetsis the formation of fibre reinforced composites using, e.g. orientedpolyethylene fibres in order to reinforce the polymer matrix. Themanufacture of such composites is a complex operation and in particularrequires careful mixing of the polymer and the fibres if the compositeis to exhibit homogeneous mechanical properties.

There have been proposals to produce polymeric sheets by compression ofnetworks of polymer fibres at elevated temperatures most notably inrelation to thermotropic liquid crystal polymers. European Patent 354285and U.S. Pat. No. 4,384,016 both describe processes in which fibres of aliquid crystal polymer are hot pressed to produce an oriented polymersheet. European Patent Application 116845, describes a process in whicha network of fibres of ultra-high molecular weight polyethylene are hotcompressed to form polymer sheets. In the processes taught in thisdocument the fibres are compressed and heated simultaneously. Theproducts retain a significant proportion of the properties of the fibresin the direction in which the fibres are aligned but the mechanicalproperties of the products in the direction transverse to that in whichthe fibres are aligned is less than ideal. These processes arerelatively unaffected by the choice of compaction temperature. Thepolymer fibres do not melt during the process.

We have now discovered a novel process whereby an assembly of fibres oforiented polymer may be hot compressed to form a sheet having superiormechanical properties particularly in the direction transverse to thatin which the fibres are aligned. The novel processes are distinguishedfrom those of EPA 116845 by an initial processing step in which thefibres are brought to and held at the compaction temperature whilstsubject to a pressure sufficient to maintain the fibres in contact, thecontact pressure, and thereafter compacted at a higher pressure, thecompaction pressure. In the processes of this invention the compactiontemperature does influence the mechanical properties of the compactedproduct. In the processes of this invention a proportion of the polymermaterial in the fibres melts and subsequently recrystallises and it isthis melt phase which serves to bind the fibres together.

Accordingly from one aspect this invention provides a process for theproduction of a polymer sheet in which an assembly of oriented polymerfibres is maintained in intimate contact at an elevated temperaturesufficient to melt a proportion of the polymer and subsequentlycompressed so as to produce a coherent polymer sheet.

In the preferred processes of this invention the conditions and moreparticularly the temperature at which the fibres are compacted will besuch as to cause a portion of the polymer to be selectively melted. Oncooling the molten materials recrystalise to give a phase with a lowermelting point than the original fibre. The presence of a second phase inthe compacted product may readily be detected e.g. by DifferentialScanning Calorimetry (DSC) measurements. In general the amount ofmaterial melted is preferably at least 5% and usually at least 10% ofthe original. The applicants believe that this minimum amount isrequired in order fill the spaces between fibres upon compaction andhence produce a product which does not contain trapped air. Processes inwhich a greater proportion of the polymer material is melted at thecontact stage are useful in so far as the mechanical properties of theproduct in the direction transverse to the alignment of the fibres maybe improved but this improvement is achieved at the expense of theproperties in the direction of the alignment of the fibres. We havediscovered that the improvements in the transverse direction are notdirectly proportional to the losses in the direction of alignment andthat the loss is greater than the improvement. For most applications ofthe products of this invention the preferred processes are those whichare carried out in a manner which selectively melts from 5 to 10% byweight of the polymer material although processes which melt from 10 to20% by weight of the polymer or even up to 50% by weight may be useful.

In a preferred embodiment the temperature at which the fibres areconpacted is not greater than the peak temperature of melting i.e. thetemperature of which the endotherm measured by Differential ScanningCalorimetry (DSC) of the polymer fibres reaches its highest point. Theminimum temperature at which the fibres should be contacted ispreferably that at which the leading edge of the endotherm extrapolatedto zero intersects the temperature axis.

The pressure at which the assembly of fibres is maintained during thisstage of the process will be such as to maintain the individual fibresin intimate contact but not such as will compact them and in particularnot inhibit the selective melting of the polymer. In general pressuresin the range 0.5 to 2.0 MPa are preferred. The precise value is notnormally critical.

The compaction pressure exerted upon the heated assembly of orientedpolymer fibres should be sufficient to produce a homogeneous product butshould not be so great as to cause the assembly to be extruded. Ifnecessary a closed mould may be used to prevent extrusion and thusallows the use of higher temperatures or pressures if required. Ingeneral, pressures in the range of 40 to 50 MPa have been found to beuseful. The minimum pressure required to process an assembly of aparticular polymer fibre at a particular temperature may be determinedby routine experiment.

The time required for the processes of this invention may be determinedby empirical means. The time required to bring the assembly of fibres upto the requisite temperature will vary with the nature and size of theassembly, the nature of the polymer and the heating means which areemployed. The time is not critical provided it is sufficient to enablethe selective melting to be achieved.

The time required for the compaction step is also non-critical except inso far as it must be sufficiently long to enable the assembly to becompacted. At the preferred temperatures the minimum time may be of theorder of seconds although longer times may be utilised. Processes whichutilise shorter compaction times e.g. 5 to 30 seconds may beadvantageous in so far as they may conveniently be operated upon acontinuous basis for example a uniaxially aligned assembly of heatedfibres may be passed between a pair of rollers.

The products of the processes of this invention preferably retain atleast 50% and more preferably at least 75% of the mechanical properties,especially the modulus of the oriented fibres in the direction in whichthose fibres are aligned. The products exhibit a homogeneous appearanceto the eye. Products which when stressed in the direction transverse tothat in which the fibres are aligned fibrillate, i.e. break whilstleaving the polymer fibres essentially intact are not homogeneous. Theproducts of this invention exhibit homogeneous behaviour when stressedin this transverse direction. Preferably they will be such that theattenuation of an ultrasonic C scan shows not more than a 20% variationand preferably not more than a 10% variation over the whole sample.

The assembly of oriented polymeric fibres which may be utilised in theprocesses of this invention may take a variety of forms. In particularthey may be arranged as an uniaxially aligned bundle or a twisted bundleof fibres or an assembly of chopped fibres or as a mat of interwovenbundles or a mat formed by layering of bundles of fibres wherein thebundles in each layer are aligned at an angle, e.g. convenientlyperpendicular to one another. The products obtained by processing suchmats may thus retain the majority of the properties of the orientedfibres in more than one direction. The bundles may be assembled andpressed into any convenient shape. The products may be flat sheets,rods, bars, any of which may be shaped so as to be suitable forparticular applications.

The oriented polymer fibres may be obtained by any of the knownmanufacturing processes. In particular, fibres which have been producedby melt spinning and drawing and gel spinning and drawing. Typicallysuch fibres will have a diameter in the range 0.005 to 0.05 mm.

The processes of this invention may be carried out using conventionalequipment. Conveniently, the fibre assembly may be placed in a suitablemould and placed under contact pressure. The assembly may then bepreheated to the desired temperature at such a rate as to ensure thatthere is no significant temperature gradient across the assembly. Thedesired compaction pressure is then applied and maintained forsufficiently long for the fibres to cohere. The hot compacted materialsare preferably cooled to ambient temperature under controlledconditions. Rapid cooling is less preferred. The most convenienttechniques is to allow the compacts to stand in the air until they havecooled to ambient temperature.

The processes of the present invention may utilise any polymer fibreswhich can be selectively melted. The susceptibility of particularpolymers and particular grades of that polymer to selective meltingvaries and their suitability for use in the processes of this inventionmay be determined empirically.

The processes of the present invention find particular application inthe production of oriented polyolefin articles especially orientedpolyethylene articles. The polyethylene (which may be a homo orcopolymer of polyethylene) may have a weight average molecular weight Mwof from 50,000 to 3,000,000. For polyethylene articles the temperatureto which the assembly is preheated is preferably within 5° C. and morepreferably within 2° C. of the peak temperature of melting. Orientedpolyethylene products of the processes of this invention preferably havea transverse (i.e. in the direction perpendicular to that in which thefibres are aligned) strength of at least 15 MPa and more preferably atleast 25 MPa.

Gel spun polyethylenes having a weight average molecular weight of atleast 500,000 may exhibit extremely high axial tensile modulus. Thiscorresponds to an extremely high degree of alignment of the polymermolecules within the fibres. These highly oriented gel spun materialsmay be processed according to this invention and may be preferred whereit is desired to produce a product which exhibits high strength in thedirection of the fibre alignment. However the strength in the directiontransverse to this alignment may be limited unless relatively highproportion of the axial strength is sacrificed by allowing the polymerto melt. Polymer fibres which are not so highly oriented may bepreferable in so far as the selective melting which characterises theprocesses of this invention may affect the axial properties to a lesserdegree whilst producing useful strengths in the transverse direction.

Homo and co polymers of polyethylene having a weight average molecularweight of from 50,000 to 500,000 particularly those which can beproduced by melt-spinning from a preferred raw material for use in theprocesses of this invention. Such polymers appear to be more amenable tothe selective melting process either by virtue of their comprising somepolymer having a relatively low molecular weight or by virtue of theirhaving a surface layer which melts at a lower temperature. Whatever themechanism which is involved those polymers are preferred because theycan form compacts which retain a large proportion of the properties ofthe fibre (in the direction of alignment of that fibre) whilst producingproducts having superior properties in the direction transverse to thatalignment.

Other classes of polymer fibres which may be useful in the processes ofthis invention include any of the known orientable polymers. Inparticular the oriented polymer may be an unsubstituted or mono or polyhalo substituted vinyl polymer, an unsubstituted or hydroxy substitutedpolyester, a polyamide, a polyetherketone or a polyacetal. Suitableexamples include vinyl chloride polymers, vinyl fluoride or vinylidenefluoride polymers PHB, PEEK and homo and copolymers of polyoxymethylene.Particular examples of polyesters useful in the processes of thisinvention include those derivable by the reaction of at least onepolyhydric alcohol, e.g. a linear polyhydric alcohol preferably a diolwith at least one poly basic acid, suitably a polycarboxylic acid. Thealcohol is preferably an alicyclic or aliphatic alcohol such ascyclohexane-dimethanol or a linear alkylene diol such as ethyleneglycol, 1,3 propylene glycol or 1,4 butylene glycol. The preferred acidsinclude o, m or ter phthalic acids, 2,6 and 1,5 napthalene dicarboxylicacid and 1,2 dihydroxy benzoic acid.

The compacted products of the present invention normally have a densityless than that of the original fibre. This reduction is caused primarilyby the retention of air within the compacted material but also by anyreduction in the content of crystalline material within the polymercaused by any molten polymer cooling to form an amorphous phase. Boththese factors detract from the properties of the product and thepreferred processes of this invention produce products in which thedensity is at least 90% more preferably at least 95% and most preferablysubstantially the same as that of the polymer fibre. This reflects thefact that the compaction should preferably be carried out in a mannerwhich expels any trapped air from the product and that in the morepreferred embodiment the compact will be cooled in a manner whichresults in the molten material forming a crystalline phase on cooling.

The processes of this invention enable complicated and precisely shapedpolymeric articles having high stiffness and high strength to bemanufactured. The products may also exhibit good energy absorbingproperties. The products find use in a wide variety of applications,particular examples being as orthodontic brackets, as bone implants andas high impact energy absorbing materials, e.g. in body armour.

The invention is illustrated by the following examples:

The tests used in these examples are defined as follows:

The fibre modulus and strength were measured on a 20 cm long sample at adisplacement rate of 20 cm/min.

The flexure modulus of the samples produced from the process weremeasured under the guidelines of ASTM D790.

The flexure strengths of the samples produced from the process weremeasured under the guidelines of ASTM D790.

The short beam shear strength of the samples measured under theguidelines of ASTM D2344.

The densities of the compacted materials were measured using a densitybottle.

Ultrasonic elastic properties were measured using an immersion method ata frequency of 2.25 MHz. A full description of the technique can befound in S. R. A. Dyer, D. Lord, I. J. Hutchinson, I. M. Ward and R. A.Duckett, J. Phys. D:Apply. Phys. 25 (1992) 66.

The fibres used were polyethylene fibres having the followingparticulars:

    __________________________________________________________________________                                   Tensile                                                    Molecular     Breaking                                                                           initial                                                    Weight        Strength                                                                           secant                                                                            modulus 2%                                 Sample                                                                            Fibre   Mw   Mn  Process                                                                            GPa  GPa GPa                                        __________________________________________________________________________    1   CELANESE                                                                                61,000                                                                           28,000                                                                            melt spun                                                                          1.0  54  36                                         2   SNIA FIBRE                                                                               130,000                                                                         12,000                                                                            melt spun                                                                              1.3                                                                                       43                                  3   TEKMILON                                                                                   700,000                                                                       54,000                                                                            solvent spun                                                                        2.1            70                                  4   SPECTRA 1000                                                                              1,500,000                                                                      75,000                                                                            gel spun                                                                              2.9                                                                                       115                                  __________________________________________________________________________

EXAMPLES

The invention will now be described in more detail with reference to thefollowing working examples.

Example 1

A sheet of dimensions 3 mm×5 cm×10 cm was prepared by hot pressing aunidirectionally aligned bundle of melt spun SNIA high moduluspolyethylene fibres having a diameter of 0.015 mm in an open endedmatched metal mould. The fibres were preheated for 10 minutes undercontact pressure of 0.5 MPa at 139±0.5° C. and then a pressure 400 MPawas applied for 10 seconds. The resulting product was a homogeneoustranslucent sheet with the following properties.

    ______________________________________                                        Tensile modulus in fibre direciton                                                              57     GPa     measured                                     Transverse to fibre direction                                                                   4.2    GPa     ultrasonically                               Flexure modulus in fibre direction                                                              35     GPa     ASTM D790                                    Transverse to fibre direction                                                                   3.2    GPa                                                  Short beam shear strength                                                                       29     GPa     ASTM D2344                                   Flexure strength in fibre                                                                       110    MPa     ASTM D790                                    direction                                                                     Transverse to fibre direction                                                                   31     MPa                                                  ______________________________________                                    

An ultrasonic immersion `C` scan of the product showed only a 2% changein attenuation over the sample and is taken as a measure of thehomogeneity of the product.

A DSC trace of the compacted material showed that 8% of the originalfibre phase had been melted and had recrystallised forming a secondlower melting point phase.

The density of the compacted material was 90% of the original fibredensity.

Example 2

A bar of 3 mm square cross section was prepared by hot pressing atwisted bundle of melt spun SNIA high modulus polyethylene fibres havinga diameter of 0.05 mm in an open ended matched metal mould. The fibreswere preheated at 139±0.5° C. for 10 minutes and then pressed for 30seconds at a pressure of 50 MPa. The resulting product was a homogeneoustranslucent bar with a flexural modulus (ASTM D790) of 32 GPa.

Example 3

An orthotropic material was made by compacting a number of layers of awoven mat of melt spun SNIA high modulus polyethylene fibres in an openended matched metal mould. The laminated mat as maintained at 139±0.5°C. for 10 minutes at 0.5 MPa before applying a high pressure of 50 MPafor 30 seconds. The flexure modulus was the same in both the axes in theplane of the plate, with a value of 11 GPa. The flexure strength wasalso similar in the two axes in the plane of the plate with a value of85 MPa. We can conclude that using a woven mat for compaction results ina substantial improvement in transverse strength at the expense ofstiffness.

Example 4

A three dimensional shape was formed by compacting a number of layers ofa woven mat of melt spun SNIA high modulus polyethylene fibre betweenmale and female hemispherical moulds. The compaction conditions wereidentical to those shown in example 3. The compacted material was formedinto the required shape in a single process.

Example 5

A laminated sheet 3 mm thick and 55 mm square was made by sandwiching auniaxially aligned bundle of melt spun SNIA polyethylene fibres betweentwo layers of a woven mat of melt spun SNIA polyethylene fibres. Thesandwich was then compacted using conditions given in example 3. Theresult was a translucent sheet with the following properties.

    ______________________________________                                        Tensile modulus in fibre direction                                                              52     GPa     measured                                     Transverse to main fibre direction                                                              4.9    GPa     ultrasonically                               Flexure modulus in main fibre                                                                   18     GPa     ASTM D790                                    direction                                                                     Flexure strength transverse to                                                                  75     GPa     ASTM D790                                    main fibre direction                                                          ______________________________________                                    

Lamination allows a better compromise to be achieve between stiffnessand strength, especially in tension.

Example 6

2.0 grams of chopped melt spun SNIA high modulus polyethylene fibre wasplaced in a cylindrical mould which was 12 mm in diameter and 30 mmlong. Compaction of the fibre assembly proceeded according to theconditions described in example 3. The resulting cylindrical bar was anisotropic material having a modulus of 5 GPa. A DSC trace of the productshowed that 12% of the original fibre had been melted.

Example 7

A bar of 25 mm square cross section and 100 mm long was prepared by hotpressing a number of cold compacted layers of melt spun SNIA highmodulus polyethylene fibres in a closed matched metal mould usingconditions described in example 3. DSC traces taken through thecompacted blocks showed that a reasonably even heat distribution hadbeen achieved.

Example 8

3.0 grams of melt spun CELANESE high modulus polyethylene fibre with adiameter of 0.015 mm was compacted in an open ended rectangular sectionsteel mould at a compaction temperature of 134±0.5° C. A contactpressure of 0.5 MPa was held for 10 minutes and then a pressure of 40MPa was applied for 30 seconds. The sample had the appearance of a solidpolyethylene rod with a well defined cross section measuring 3.34mm×3.11 mm. The bending modulus was 19.7 GPa.

Example 9

To demonstrate the criticality of the moulding temperature, a sampleidentical to that used in example 8 was compressed in the same mould atthe higher temperature of 138° C. The resulting sample again had theappearance of a solid polyethylene rod but the low bending modulus of1.2 GPa showed that the properties of the fibre had been lost due tosubstantial melting of the original fibre phase. Further evidence of thecritical nature of the temperature was shown by compressing an identicalsample to examples 8 and 9 but at the lower temperature of 127° C. Theresulting product had a high stiffness but poor transverse propertiesdue to almost total retention of the original fibre phase.

Example 10

The role of pressure was examined by carrying out an identicalexperiment to example 1 except that high pressure (40 MPa) was appliedfrom the very start of the procedure, including the warm up period. Theresulting product had a high longitudinal stiffness of 60 GPa but a poortransverse strength of 12 MPa. A DSC trace of the compacted materialshowed no evidence of any `second phase`: the compacted material wascomposed entirely of the original fibre phase.

We can therefore conclude that applying high pressure from the beginningof the compaction process inhibits the selective melting which isnecessary for optimum control of the properties of the final product.

Example 11

A sheet of dimensions 3 mm×55 mm×55 mm was prepared by compacting aunidirectionally aligned bundle of gel spun SPECTRA high moduluspolyethylene fibres in a matched metal mould. The processing conditionswere identical to example 3 apart from raising the compactiontemperature to 152±0.5° C., which is midway between the onset of meltingand the end of melting.

The resulting compacted sheet was homogeneous and had a longitudinalmodulus of 35 GPa and a transverse strength of 17 MPa. A DSC trace ofthe compacted material showed around 35% of a `second phase` formed bymelting of the original fibre.

We claim:
 1. A homogeneous polymeric product produced from molecularlyoriented melt spun or gel spun fibers of thermoplastic polymer, saidproduct being produced by a process comprising the steps of:forming anassembly of molecularly oriented thermoplastic melt spun or gel spunfibers of thermoplastic polymer; applying a contact pressure to saidassembly sufficient to ensure intimate contact by the polymer fiberswith each other; heating said assembly of polymer fibers to an elevatedtemperature sufficient to selectively melt between 5 and 50% by weightof the polymer fibers while in intimate contact with each other, themolten polymer on cooling recrystallizing to form a melt phase which hasa melting point less than the melting point of the fibers and whichbinds said fibers together; and subsequently compressing said boundfibers at a compaction pressure higher than said contact pressure whilestill maintaining said assembly at an elevated temperature to producesaid product, said product having higher strength in a directiontransverse to said fibers as compared to a product produced withoutapplication of a contact pressure.
 2. A product according to claim 1,wherein said thermoplastic polymer fibers are selected from the groupconsisting of homo- and copolymers of a polyolefin.
 3. A productaccording to claim 1, wherein said thermoplastic polymer fibers arepolyethylene fibers.
 4. A product according to claim 1, wherein saidthermoplastic polymer fibers are fibers of a polymer selected from thegroup consisting of a vinyl polymer, a polyester, a polyamide, apolyetherketone and a polyacetal.
 5. A product according to claim 1,wherein said polyester is polyethylene terephthalate.
 6. A product asclaimed in claim 1, wherein the molecularly oriented polymer fibers arefibers that have been melt spun or gel spun and then drawn.
 7. A productas claimed in claim 1, wherein the thermoplastic polymer is apolyolefin.
 8. A product as claimed in claim 1, wherein therecrystallized melt phase is derived from between 5 to 20% by weight ofthe molecularly oriented polymer fiber.